Implement PRE of loads in the GVN pass with a pretty cheap and

straight-forward implementation.  This does not require any extra
alias analysis queries beyond what we already do for non-local loads.

Some programs really really like load PRE.  For example, SPASS triggers
this ~1000 times, ~300 times in 255.vortex, and ~1500 times on 403.gcc.

The biggest limitation to the implementation is that it does not split
critical edges.  This is a huge killer on many programs and should be
addressed after the initial patch is enabled by default.

The implementation of this should incidentally speed up rejection of 
non-local loads because it avoids creating the repl densemap in cases 
when it won't be used for fully redundant loads.

This is currently disabled by default.
Before I turn this on, I need to fix a couple of miscompilations in
the testsuite, look at compile time performance numbers, and look at
perf impact.  This is pretty close to ready though.




git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@60408 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 18 insertions, 0 deletions

diff --git a/test/Transforms/GVN/pre-load.ll b/test/Transforms/GVN/pre-load.ll
new file mode 100644
index 0000000..bc3ec5b
--- /dev/null
+++ b/test/Transforms/GVN/pre-load.ll
@@ -0,0 +1,18 @@
+; RUN: llvm-as < %s | opt -gvn -enable-load-pre | llvm-dis | grep {%PRE.rle = phi}
+
+define i32 @test(i32* %p, i1 %C) {
+block1:
+	br i1 %C, label %block2, label %block3
+
+block2:
+ br label %block4
+
+block3:
+  %b = bitcast i32 0 to i32
+  store i32 %b, i32* %p
+  br label %block4
+
+block4:
+  %PRE = load i32* %p
+  ret i32 %PRE
+}